The instant invention relates to thermal plotting apparatus and more particularly to a wide-format thermal plotter utilizing multiple print heads to achieve a wide print web.
Multi-head, wide-format thermal plotting devices have heretofore been known in the art. In this regard, the U.S. Patents to Kaiya et al U.S. Pat. No. 4,660,052 Onuki et al U.S. Pat. No. 4,977,410 and Onuki et al U.S. Pat. No. 5,003,323 represent the closet prior art to the subject invention of which the applicant is aware. The patents to Kaiya and Onuki et al each disclose a thermal recording apparatus wherein plurality of thermal print heads (approximately 8.5 in wide) are alternately arranged in two successive rows over two parallel platen rollers. The print lines of the print heads are stitched together to provide coverage across the entire width of the print media. With appropriate data buffering a single print line having an effective width which is wider than any one individual print head can be achieved. While these apparatus are relatively effective for wide format printing, they have several drawbacks. Since the print heads are alternately arranged over two spaced parallel platen rollers, the printing apparatus must include an elongated flat print bed in order to accommodate the linear distance between the spaced platen rollers. Since each staggered row has its own platen roller, the rollers are passive rollers. Such an arrangement requires a pair of drive rollers situated downstream of the last platen roller for drawing the print media through the successive print stations. Accordingly, the print bed must further accommodate the distance between the last platen roller and the drive rollers. It can thus be seen that these types of print apparatus are extremely large occupying a significant amount of office or desk space. In order to reduce the distance between the two parallel platen rollers, the Onuki patent ('323) utilized near edge print heads mounted such that upstream and downstream near edge print lines oppose each other. However, the space saving in Onuki ('323) is minimal at best. Another significant problem is maintaining parallelism of the two platen rollers in all three axes. Slight deviations in parallelism will result in inaccurate stitching of the print lines and inaccurate paper tracking. Accordingly, the parts of these machines must be produced with very low tolerance factors in order to achieve almost perfect parallelism. Such manufacturing adds to the expense of the machines. Yet another problem is maintaining sufficient stiffness of the platen roller across the width of the print media. The structural limitations of conventional print heads require the use of relatively small-diameter platen rollers. However, it has been found that a narrow platen roller will invariably bow under pressure of the print heads when supported across a wide span. The U.S. Patent to Tzeng et al U.S. Pat. No. 4,916,463 has attempted to address this problem by supporting the platen roller with two spaced support rollers. However, this solution requires the mounting of at least four additional rollers in the prior apparatus thereby increasing cost and complexity of the apparatus.
The above-mentioned disadvantages are overcome in the instant invention by providing a wide-format thermal printing apparatus comprising a single platen roller and a plurality of thermal print heads arranged in first and second diametrically opposed rows on the platen roller. The rows of print heads in the instant device are arranged in upper and lower opposed relation so that the first, lower row supports the center of the platen roller to prevent bowing thereof. In the preferred embodiment, the first lower row includes two spaced print heads and the second upper row includes three spaced print heads. A supply of print media is sequentially threaded through the first and second rows of print heads, wrapping around the platen roller so that the second, upper row is the downstream printing row. The print heads are arranged in staggered relation between the rows such that print lines of print heads in alternate rows abut or overlap each other with respect to the width of print media. Each print head is mounted for pressured engagement with the platen roller wherein the print heads in the second row are adjusted to have a greater engagement pressure than the print heads in the first row. A back tension arm engages the print media roll for providing back tension to the media supply. The printing apparatus further includes a motor, timing belts, pulleys, and gears for driving the single platen roller. The combined print head pressures in both rows cooperate to form a nip to sequentially draw the print media through the first and second rows of print heads. It is noted however, that the engagement pressure of the first row by itself creates insufficient drawing force to overcome the media back tension. This arrangement assures a tensioning force between the first and second rows of print heads and ensures that the print media remains in contact with the platen roller at all times. The higher engagement pressure of the second row of print heads assures sufficient pulling force to pull the media through the print stations.
Accordingly, among the objects of the instant invention are: the provision of a low cost, wide-format thermal plotter; the provision of a wide-format thermal plotter which is smaller than the prior art devices; and the provision of a wide-format thermal plotter having a plurality of standard width thermal print heads mounted in two staggered rows at diametrically opposite positions on a single platen roller.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.